Representing local accumulations of minor defects in ductile cast iron by an appropriate material model

M. Suty, H.-C. Schneider and O. Kraft

Karlsruhe Institute of Technology, Institute for Materials Research,

P.O. Box 3640, D-76021 Karlsruhe, Germany

Mechanical components in nuclear engineering are subject to strong safety requirements. Especially thick-walled castings require appropriate mechanical testing and Finite Element calculations. A crucial point is the consideration of minor defects. Those are difficult to avoid consistently but do not necessarily affect the structural integrity of castings. By now, local accumulations of such defects are conservatively enveloped and replaced by one large defect with a well known geometry.

In general, material parameters are obtained by testing tensile and impact specimens taken out of the cast. The challenge is to extract a set of significant test specimens according to the licensing and testing standards without unduly weakening the casted component.

In this work, an alternative approach is described for considering cast defects such as voids in ductile cast iron or porous plastic metals. A material model successfully used to homogenize porous structures is adapted to the mechanical behaviour of the affected area which is included in calculations.

In order to validate the model spherical defects are represented in two dimensions by drilled holes (1.2 mm) in flat bar tension specimens, with sufficient sample dimensions to avoid boundary effects (60x100x4 mm³). Finite Element calculations furnish stress distributions in the experiments. The micromechanical Gurson-Tvergaard-Needleman model is applied to represent the behaviour of the cast iron with the holes. This approach still gives a conservative description of the influence of small voids or other defects on the safety of large casting components for defect dimensions up to 100 times the size of the graphite spheres.

First results of tensile tests will be presented. The drilled holes reduce the tensile strength by 5 % from 380 MPa to 360 MPa considering a net cross-sectional reduction of 6 %. The implications of these findings are discussed in the context of the described model.